The invention relates to an apparatus and method for determining a non-condensable gas parameter relating to the amount of non-condensable gas within a fluid flow containing both non-condensable gas and condensate.
The presence of non-condensable gas, such as air, within a steam flow can significantly alter the properties of the steam flow. For example, non-condensable gas may affect the heat transfer properties of the steam flow. Further, non-condensable gas may lead to heterogeneous steam properties within the steam flow, such as regions of superheated steam and regions of saturated steam.
In a number of industrial processes, it is desirable to minimize the amount of non-condensable gas within a steam flow. One such example is the sterilization of products and/or equipment in a steam flow, for example in the medical and pharmaceutical industries. If the amount of non-condensable gas within the steam flow is too high, the heat transfer rate between the steam flow and the equipment may be insufficient to properly sterilize the equipment.
It is known to measure the amount of non-condensable gas in a steam flow by condensing a sample of the steam and collecting the resultant non-condensable gas and condensate. European Standard EN 285 defines a safety limit of 3.5 ml of non-condensable gas per 100 ml of condensate, or 3.5%, for sterilization applications.
Non-condensable gas may be introduced into a steam system in a number of ways. For example, non-condensable gas can be introduced by excessive aeration during water treatment, such as water softening. Further, non-condensable gas can be introduced into a steam system upstream of a boiler if the inlet water for the boiler is not pre-heated sufficiently, since non-condensable gas (e.g. air) is more easily absorbed in water at lower temperatures. Insufficient pre-heating may occur in a variable demand steam system when the amount of steam demanded from the boiler is increased, therefore increasing the flow rate of inlet water through the pre-heater to the boiler.
U.S. Pat. No. 4,831,867 discloses a method and apparatus for estimating the proportion of non-condensable gas within a steam flow by extracting a portion of the steam flow and condensing it to provide a fluid flow of substantially constant flow rate containing bubbles of non-condensable gas and drops of liquid condensate. An optical sensor records the cumulative time that non-condensable gas bubbles pass the sensor, and the cumulative time that condensate drops pass the sensor. It is assumed that the ratio of the respective lengths of the gas and condensate passing the sensor are constant, and that the flow rate is substantially constant. Accordingly, the method and apparatus only compares the cumulative times of gas and condensate flowing past the sensor in order to estimate the proportion of non-condensable gas within the steam flow.
However, the estimate of the proportion of non-condensable gas may be inaccurate where the flow rate of the fluid flow is variable, and where the respective steam flow has a varying proportion of non-condensable gas.
Accordingly, it is desirable to provide an improved apparatus and method for measuring the amount of non-condensable gas in a fluid flow.